The present application claims priority of Chinese patent application Serial No. 200610171670.5, filed Dec. 31, 2006, the content of which is hereby incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a technology of measuring terahertz time-domain spectrum, and in particularly, to a method and an apparatus for measuring terahertz time-domain spectrum by using femtosecond pulsed laser to generate and detect the terahertz (hereinafter referred as “THz” for short) pulses.
2. Description of the Prior Art
As the technology of generation and detection of terahertz waves (whose waveband ranges from 0.05 THz to 50 THz, particularly electromagnetic waves ranges from 0.1 Thz to 10 THz) grows up, relevant technology and application research of terahertz waves develop rapidly. Terahertz time-domain spectroscopy technology is one of the most important technologies in terahertz-wave research. Terahertz reflection or transmission spectrum of a detected object can be obtained by terahertz time-domain spectroscopy technology, and then the spectrum obtained can be used for discriminating composition of the object, thereby the technology can be widely used in various applications such as quality detection, security inspection, antiterrorism and the likes.
Document 1 (ZHANG XingNing, etc., Terahertz Time-domain Spectroscopy Technology, LASER AND OPTOELECTRONICS DEVELOPMENT, July, 2005, p 35˜38) discloses a terahertz time-domain spectroscopy measuring method which uses a femtosecond laser device to generate a femtosecond laser beam, and the generated femtosecond laser beam is divided into two beams. One beam acts as a pumping beam to activate the terahertz laser device to generate terahertz pulses, and the other one acts as a detecting beam incident on the terahertz laser detector to measure the terahertz electric field intensity of the detected beam at arrival. When the optical path difference between the two propagation paths is constant, the generated terahertz pulses corresponding to each laser pulse have a constant delay relative to the detecting pulses. Therefore, only one point of the terahertz pulses in the time axis is detected by the detecting pulses. Terahertz electric field intensity at different time point in the time axis can be detected by a set of precise mechanical displacement means which can adjusts the propagation path of one beam (usually the detecting beam) to change the propagation path difference there between, and then the time-domain waveform of the amplitude of the terahertz pulses can be obtained. After that, a spectrogram (time-domain spectroscopy) of the terahertz pulses can be obtained by performing Fourier transformation of the intensity data of the pulses.
However, such conventional terahertz time-domain spectroscopy method uses mechanical time-delay means. It is difficult to make measurement over a broad time window (for example, 1 ns or even larger than 1 ns) because movement of the mechanical means inevitably changes the path (including size of faculae, displacement of the position and so on), and the larger the movement is, the more the optical path changes, so the spectroscopy resolution thereof is limited (the typical value is 3-50 GHz). Furthermore, the scan speed of the system which is based on mechanical time-delay means is slow. In the case, the spectroscopy resolution has to be sacrificed in order to improve the scan speed.
Document 2 (A. Bartels, etc., High-resolution THz spectrometer with kHz scan rates, OPTICS EXPRESS, Vol. 14, NO. 1, p 430˜437) discloses a THz time-domain spectroscopy method which is a time-domain spectroscopy method of asynchronous optical sampling. In the method of Document 2, two femtosecond laser devices operating at different repetition frequencies are used to generate two femtosecond laser beams. The two laser beams generated by the two laser devices are used as a pumping beam and a detecting beam respectively. Unlike the Document 1 in which the THz time-domain spectroscopy measuring method requires mechanical time-delay means to adjust the delay between the pumping pulses and the detecting pulses, in the method of Document 2, the delay between the pulses of the two beams is always changed because these two beams operate at different repetition frequencies. Let the repetition frequency of the pumping beam be f, the frequency difference between the two lasers be Δf, then the detecting pulses scan the THz pulses one time in a time window of 1/f. The signal noise ratio can be improved through repetition scans, and the time-domain waveform can be obtained finally. As such, a spectrogram of the terahertz pulses can be obtained by performing Fourier transformation of the pulse intensity data.
As described above, the THz time-domain spectroscopy method based on asynchronous optical sampling can omit the mechanical time-delay means, and efficiently eliminate the conflict between the scan speed and the spectroscopy resolution, thus the system can operate at a high scan speed (the typical time of a single scan is 0.1 ms, and the typical SNR of multiple scans is 60 dB@60 s) while have a high spectroscopy resolution (the typical value is 1 GHz). However, the method increases the repetition frequency of the femtosecond laser (typically, to 1 GHz from 80 MHz) in order to guarantee the measuring bandwidth and solve the frequency stability problem, which makes the spectroscopy resolution can not be further improved (the theoretical spectroscopy resolution of 1 GHz repetition frequency is 1 GHz). Moreover, in order to broader the detection bandwidth, the stability of the laser repetition frequency should be improved, but it is very difficult to further improve the stability of the laser repetition frequency.